1. Field of the Invention
The present invention relates generally to battery-powered, power tools, and more specifically to torque and speed management systems for such tools.
2. Description of Related Art
Power tools (e.g., nutrunners) such as those used for securing threaded fasteners (e.g., nuts, bolts, screws, etc.) are typically selected by their ability to tighten the threaded fastener to a specified torque level. Such tools typically have a high-speed, low-torque motor coupled to a speed reduction transmission in order to increase the output torque to the desired tightening torque level. The output of this speed reduction transmission may in turn couple to an output head so configured to suit the intended use; e.g. an angle or offset output head may be used to allow access to specific threaded fasteners. Various interchangeable bits or sockets may connect to the output head in order to drive threaded fasteners, e.g., bits or sockets appropriate for driving hex-head bolts and hexagonal nuts. Various methods of limiting or controlling the tightening torque may be employed to suit the intended use. The speed reduction transmission has a gear ratio sufficient to increase the output torque of the motor to a level equal to or greater than the specified tightening torque level for a particular fastener. This gear reduction transmission will also reduce the output spindle speed by the same ratio.
The process of securing a threaded fastener typically consists of two distinct phases: a first, relatively free-running, or low torque resistance phase whereby the fastener is rotated through a number of revolutions to engage the fastener threads with the threads of the mating part, and a second, relatively high torque resistance tightening phase whereby the mechanical advantage of its screw threads are used to clamp together the mating parts and preload the fastener. Depending on the dimensional characteristics and mechanical properties of the mating parts of the assembly, the first phase may include roughly 3 to 10 revolutions (or more or less depending on the particular application), whereas the second phase may consist of less than a single revolution. A power tool suitable for securing threaded fasteners must be capable of performing both phases of the securing process. When selecting a motor for use in a power tool intended for securing threaded fasteners, a manufacturer typically compromises performance in one phase in order to achieve the desired performance in the other phase. That is, a manufacturer may sacrifice speed to increase tightening torque capacity or vice-versa.
Battery-powered tools are limited by the available energy stored in the battery pack and by the electrical characteristics of the motor. Battery-powered tools are typically intended to be portable, thereby limiting the practical size of the motor and the battery pack. Smaller motors and lighter battery packs will typically result in lower motor torque characteristics. Motors with low output torque characteristics will require a high gear ratio to produce the required output torque resulting in a low output spindle speed. For battery-powered tools intended for production use, an excessively low output spindle speed will negatively affect productivity.
Battery-powered tools are also limited by the rate at which the battery pack can deliver its stored energy to the motor. This is a function of the voltage and current capacity of the battery pack. Typically, the battery voltage level will drop as more current is delivered to the motor. Battery cell chemistry affects its ability to deliver current to the motor with each particular cell chemistry having its practical limits.